Aluminet shade cloth is a great choice! What percentage can be tricky as most Vanda's enjoy bright light but some don't, when I had mine I had some that were in direct sunlight and thrived. The best way to determine is to check where the sun is hitting from mid morning till mid afternoon that is the time the sun can do the most damage. Personally I use 70% but I am in south Florida and most of what gets sun is my area where my Bulbophyllum are kept and most don't like harsh sunlight. As to when you put the shade cloth on and take it down when you are putting your Vanda's outdoors put it up when you take them down take the shade cloth down.